1. Field of the Invention
The present invention relates to an access network system that is provided with a plurality of external network relay devices, which are relay devices for connecting with other networks, and a plurality of internal network relay devices for relaying information that is transmitted and received within a network, and to a method of moving internal network relay devices for causing said internal network relay devices to change groups of affiliation in an access network system.
2. Description of the Related Art
Normally, a network such as an LAN (Local Area Network) in a business organization is configured by connecting together in tree form a plurality of internal network relay devices (such as layer-2 switches and hubs, or computers) for relaying information (such as frames or packets) that is transmitted and received between end systems such as personal computers. The occurrence of a problem in any internal network relay device in such a configuration presents a serious problem that can bring business operations to a halt because communication with server devices that are within the same network is disrupted.
As one example for avoiding such problems, redundant communication paths are formed such that an internal network relay device in which a problem has occurred can be bypassed. However, providing communication paths as a redundant configuration raises the potential for the formation of loops and the consequent inability to realize normal communication.
A control method referred to as a spanning tree is prescribed in IEEE 802.1D for preventing the perpetual circulation of frames within a network. In this method, control information known as Bridge Protocol Data Units (BPDU) is exchanged between internal network relay devices, and a topology in a logical tree shape is formed that prevents the logical use of portions of a network that form physical loops.
In addition, a control method known as a high-speed spanning tree is prescribed in IEEE 802.1w in which the method of exchanging control information is expanded to accelerate the creation of the spanning tree prescribed in the above-described IEEE 802.1D, and further, a detour path is set in advance for the rapid securing of a detour path in the event of a problem.
A method referred to as PVST (Per VLAN Spanning Tree) also exists in which a plurality of VLAN (Virtual LAN) are formed using internal network devices that conform to the standards of IEEE 802.1Q, and in which independent spanning trees are formed for each VLAN, whereby the load of the network is distributed by the appropriate use of separate communication paths.
When the spanning tree topology is independent for each VLAN as in PVST, the spanning trees that are managed increase with increase in the number of VLAN, and this situation leads to an increase in processing for the spanning trees and a huge processing load for the CPU that is provided in the internal network relay devices. To compensate for this drawback and further, for mapping any VLAN to a plurality of topologies that have been created in advance, a control method known as a multiple spanning tree is prescribed in IEEE 802.1s. This method is here referred to as the first example of the prior art. The first example of the prior art avoids loop structures that are caused by redundancy of communication paths, and further, realizes static load distribution for each VLAN.
Networks are connected so as to allow communication with other networks by way of external network relay devices (such as routers, layer-3 switches, or computers), which are relay devices for connecting to other networks. The occurrence of a problem in an external network relay device in such a configuration disrupts communication with, for example, the server devices of other networks, resulting in serious problems such as the halt of business operations.
In one example for avoiding this type of problem, a plurality of external network relay devices may be established, but in an end system such as a personal computer that lacks a dynamic path switching capability, a gateway (an external network relay device) is statically designated as a default, and a problem that occurs at the gateway is therefore impossible to handle.
In response, technologies have been proposed and realized for improving system reliability by using a plurality of routers to construct a virtual router, and then switching the routers that carry out routing in the virtual router when a problem occurs. As an example of this technology, a technology known as VRRP (Virtual Router Redundant Protocol) has been standardized by IETF (The Internet Engineering Task Force), which is an international Internet organization.
In the above-described virtual router, the plurality of routers operates by dividing between a master router that actually performs routing and a backup router that carries on processing when a problem occurs in the master router. When a problem occurs in the master router or in a path within the monitor area of the master router, the backup router executes processing in place of the master router and continues communication. This processing raises the reliability of the system. However, in the virtual router, the backup router is always in an active state of transmitting and receiving packets to verify whether a problem has occurred in the master router. However, since the backup router does not normally perform routing, full advantage is not taken of the routing capability that is provided in the devices.
In VRRP that is standardized by the above-described IETF, static load distribution is realized by forming a plurality of groups. However, no consideration is given to the dynamic load state of each router in this method, and cases may occur in which load is concentrated in only a specific router and load cannot be distributed.
To deal with such a state, a system has been proposed in, for example, Japanese laid-open patent publication No. 2003-23444, for realizing dynamic load distribution by setting conditions such that, depending on the processing load such as the amount of flow of packets, the master router makes backup routers route packets, and entrusts backup routers with the routing of a portion of packets. This system is here referred to as the second example of the prior art.
In the above-described first example of the prior art, static load distribution was realized by mapping any VLAN to a plurality of trees. However, the load of a network is constantly fluctuating, and there is the consequent problem that the traffic of a network cannot be maximized by static load distribution alone.
In addition, in the first example of the prior art, a communication path is not switched until an internal network relay devices becomes completely incapable of communication, and as a result, load may concentrate in a particular internal network relay device without switching to another communication path despite the concentration of traffic in the specific internal network relay device and the drastic reduction in processing efficiency.
In the second example of the prior art, in contrast, time is required for adjustments between routers when switching routers that are to carry out routing in order to prevent simultaneous routing by a plurality of routers and consequent packet loss. Accordingly, routers cannot be switched in a short time, and a particular router may therefore be subjected to a high-load state for a lengthy time interval. A router that is subjected to a high-load state for a long time not only suffers a reduction of processing capabilities but also runs a higher risk for the occurrence of a problem, whereby the reliability of the overall network drops.